1. Field of the Invention
The present invention relates to a low-loss optical fiber over wide wavelength range that has a low-loss in a broad range of wavelengths from a visible light region (400-750 nm) to a near-infrared light region (up to 1400 nm), and to a method of manufacturing the same.
2. Description of the Related Art
In an optical fiber having a low-loss at an ultraviolet region and a visible light region, high-OH silica glass doped with hydroxyl groups in high concentration is used as a constituent material of a core.
In addition, in an optical fiber used for wavelengths longer than a visible light region, in order to reduce an absorption loss (i.e., an absorption loss which occurs mainly at around the wavelength of 1.38 μm) caused by hydroxyl groups, low-OH silica glass in which a hydroxyl-group concentration is reduced is used as a constituent material of a core.
Attenuation characteristics of an optical fiber having a core composed of high-OH silica glass and of an optical fiber having a core composed of low-OH silica glass are shown in FIG. 5. Here, the hydroxyl-group concentration of the core composed of high-OH silica glass is approximately 1000 ppm, and the hydroxyl-group concentration of the core composed of low-OH silica glass is less than or equal to 1 ppm.
Fiber attenuation characteristics described in “Specialty Fiber Preforms for the Most Demanding Applications” by Heraeus Holding (hereinafter, referred to as Document 1) is shown in FIG. 6.
SSU/SBU, SOU, and SXU show lower attenuation at less than or equal to the wavelength of 500 nm; however, show higher attenuation at more than or equal to the wavelength of 700 nm due to the absorption loss of hydroxyl groups.
Regarding STU, STU-D, and SWU, the absorption loss of hydroxyl groups is relatively low, and low attenuation appears to be achieved in a near-infrared light region. However, at less than or equal to the wavelength of 500 nm, the line of the attenuation characteristics of SWU is rapidly away from the lines of the attenuation characteristics of SSU/SBU, SOU, and SXU, and the attenuation increases. In addition, at less than or equal to the wavelength of 500 nm, the lines of the attenuation characteristics of STU and STU-D are gradually away from the lines of the attenuation characteristics of SSU/SBU, SOU, and SXU, and the attenuation increases.
It is found from FIG. 6 that there are no optical fibers having attenuation of less than or equal to tens of dB/km near the wavelength of 1400 nm and attenuation of similar as SSU/SBU, SOU, and SXU at the wavelength of 400 nm.
Being applied to a broad range of wavelengths from a visible light region (400-750 nm) to a near-infrared light region (up to 1400 nm), a conventional optical fiber composed of low-OH silica glass has not been sufficiently practical since such optical fiber has a high transmission loss in a visible light region. On the other hand, being applied to a broad range of wavelengths from a visible light region (400-750 nm) to a near-infrared light region (up to 1400 nm), a conventional optical fiber composed of high-OH silica glass has not been sufficiently practical since such optical fiber has a high transmission loss in more than or equal to the wavelength of 700 nm.
Polymicro Technologies™ in the U.S., a manufacturer of specialty fibers, discloses lower loss optical fibers in a broad range of wavelengths (for example, Polymicro Technologies, LLC, “SILICA/SILICA Optical Fiber—FPB”: http://www.polymicro.com/products/opticalfibers/products_opticalfibers_fbp.htm, searched by Fujikura Ltd. on May 7, 2012, hereinafter, referred to as Document 2). Attenuation characteristics of an optical fiber over wide wavelength range disclosed in the above-described document is shown in FIG. 7.
However, even in these optical fibers, the loss at the wavelength of 1400 nm greatly increases due to the absorption loss of hydroxyl groups, and therefore, the loss further needs to be reduced.